This invention relates to an exhaust system for a marine propulsion unit and more particularly to an improved exhaust system for an outboard motor and particularly to a low speed exhaust gas discharge for such outboard motors.
The problem of exhaust gas treatment in marine propulsion units and particularly outboard motors are well known. One of the main difficulties in providing an effective exhaust system for an outboard motor is the fact that the space in which the exhaust gases may be treated is substantially limited. It is the conventional practice to provide an expansion chamber and possibly other silencing device in the drive shaft housing to which exhaust gases are delivered from the engine of the powerhead by an exhaust pipe. These exhaust gases are then discharged through a high speed underwater exhaust gas discharge which may be formed in the lower unit or in the hub of the propeller. Although such systems are relatively effective, when the outboard motor and associated watercraft are traveling at a low speed or idling, the underwater discharge becomes very deeply submerged and the exhaust gas pressures are not great enough to permit their passage through this discharge.
It is, therefor, the common practice to provide an additional, above the water low speed exhaust gas discharge. This discharge is conventionally formed in either the upper portion of the drive shaft housing or in a lower portion of the powerhead itself and communicates with either the expansion chamber or some other point in the exhaust system. When the outboard motor is operating at the low speed conditions, as aforenoted, the exhaust gases can exit through this above the water exhaust gas discharge. Of course, it is necessary to insure that this discharge is somewhat restricted so as to insure that any large portions of the exhaust gases will not flow out of this opening when traveling at other than low speeds or at idle.
The exhaust gases which exit the above the water exhaust gas discharge obviously do not have the benefit of the full silencing afforded by the underwater exhaust gas discharge. In addition, the above the water exhaust gas discharge is in closer proximity to the operator and thus there is problem of noise generation with the systems.
The noise from the above the water exhaust gas discharge can be further silenced by providing either a labyrinthian flow in the upper casing and/or by including expansion chamber volumes. However, there are disadvantages with these types of arrangements.
Specifically, if a labyrinthian type of flow is to be provided, then the drive shaft housing casting either becomes complicated or addition parts must be formed and fixed within the drive shaft housing. This is not always feasible due to the desire to maintain a relatively low and uncomplicated drive shaft housing assembly.
Even greater problems exist with attempts to provide expansion chambers since the effectiveness of an expansion chamber depends to a large extent on its volume and the configuration and size of the drive shaft housing does not permit the use of large expansion chambers.
There is also the problem attendant when the outboard motor is operating at the low speed conditions. Water will back up in the interior of the drive shaft housing and this can further reduce the available size for expansion chambers and the like unless there is some further methodology employed so as to seal the expansion chambers from this water influx, which again complicates the casting process for the drive shaft housing.
It is, therefor, a principal object of this invention to provide an improved exhaust silencing arrangement for a marine propulsion unit.
It a further object of this invention to provide an improved low speed silencing system for an outboard motor that can employ fairly large expansion chambers and/or labyrinthian type flow systems without complicating the drive shaft housing construction or substantially increasing the size of the drive shaft housing.
It is a still further object of this invention to provide an improved arrangement for providing external silencing for the low speed exhaust gas discharge of an outboard.